Optimize our tests to run faster.

Changes include:

 * default tests locale is Polish instead of Spanish

   Rationale: see commit 049f59a.

              tl;dr¹: 1 vs 20 locales to generate, that’s all.

 * flake8 is running on its own build jobs.

   Rationale: we are now able to tell what passed and what didn’t at a
              glance.  We do not have to search through stuff to find out
              what (nosetests or flake8) failed.

 * Use the Travis apt cache, which will save us a few seconds here or
   there.

¹ This commit message is actually longer than the one in 049f59a.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>